1. Field of the Invention
The present invention relates to a method of making a mask structure (or a structural body) used in X-ray lithography. More particularly, the present invention relates to a method of making an X-ray mask structure that can be fixed to an apparatus body by magnetic chucking.
2. Related Background Art
Various methods have been hitherto employed in lithographic processing of electronic devices such as IC and LSI. In particular, X-ray lithography is based on properties inherent to X-rays, as exemplified by high transmittance (or low absorbance) and short wavelength. X-ray lithography has a number of advantages when compared with conventional lithographic methods that utilize visible light or ultraviolet light, and has attracted notice as a promising method for submicroscopic lithography.
Mask structures used in X-ray lithography are commonly prepared in the following way.
First, a thin film (a support film) comprising a material having a high X-ray transmittance is formed on a holding frame for holding the film, and thereafter, an X-ray absorber, such as Au, Ta and W formed into patterns corresponding to a shape to be transformed, is formed on the film.
Because of the use of soft X-rays of about 0.5 to 2 nm in X-ray lithography, X-rays may be so greatly attenuated in the atmosphere that exposure is required to be operated in a vacuum or in low-pressure helium. Now, as a method of fitting the X-ray mask structure to an apparatus body in such X-ray lithography, a magnetic chucking method has been proposed in which a magnetic force is utilized in place of vacuum attraction, as disclosed in Japanese Patent Application Laid-Open No. 60-68340. When, however, the material of the holding frame is made of a magnetic material as proposed in the Japanese Patent Application No. 60-68340, a force based on the Lorentz force is exerted on moving electric-charged particles (Electrons), because of its residual magnetic flux (density) to cause misregistration from a given position, creating a problem that the position accuracy in drawing X-ray absorber patterns and the measurement accuracy in carrying out quality inspection on X-ray mask structures are extremely lowered.
In the above proposal, the magnetic material, such as metal, constituting at least a part of the holding frame, is apt to rust, often bringing about the problem that the rust causes parallel warpage when the mask structure is fixed to the apparatus body.